Electron Microscopy and Energy Dispersive X-Ray Analysis

Selected WTC dust samples were analyzed by scanning electron microscopy
(SEM) and energy dispersive x-ray microanalysis (EDS) at the USGS
Denver Microbeam Laboratory (exit to the Denver Microbeam Laboratory
web site by clicking here).
The primary purpose of the SEM/EDS analysis was to determine if the WTC
dust contained asbestos. Electron microscopy was used because extremely
low levels of asbestos fibers can be detected and chemically analyzed.
Representative portions of samples wtc01-3, wtc01-8, wtc01-14,
wtc01-15, wtc01-16, wtc01-20, wtc01-22, wtc01-25, wtc01-27, wtc01-28
and wtc01-36 were selected for this analysis (See sample location map for
exact collection sites).

Amphibole asbestos was not detected in the dust samples by SEM/EDS analysis.
However, trace amounts of chrysotile
asbestos have been identified in several of the samples. There are also
abundant glass fibers in all samples analyzed so far. Other phases
found in these samples include gypsum and/or anhydrite (calcium sulfate
minerals), calcium-rich phases compatible with concrete materials, and
rock and mineral fragments such as quartz and feldspar. A large variety
of other materials are present at trace levels including unidentified
organic materials compatible with wood, paper, etc., and particles
enriched in Fe, Pb, Zn, Sr, Bi, Cu and other metals.

Preliminary SEM and EDS analysis of material coating a steel beam
(sample 8) from the WTC debris indicates that chrysotile asbestos is
present at a level possibly as high as 20% by volume. So far, no
amphibole asbestos has been detected in this material. This material
also contains abundant glass fibers.

Energy dispersive x-ray microanalysis (EDS) was performed to determine
the chemical composition of selected materials in the dust.
Representative compositions of the glass fibers are given in Table
SEM-1. The chemical composition of the majority of glass fibers (and
glass spheres) in all samples is consistent with slag wool, a synthetic
fiber commonly used in building materials like ceiling tiles
(Nomenclature Committee of TIMA Inc., 1991). Glass fibers of other
compositions were also found in some of the dust samples (see Table
SEM-1, analysis wtc 22 sp2), and may indicate a different source
material. Sample 8, collected from a coating on a steel beam, contains
glass fibers, chrysotile and gypsum and/or anhydrite. The chrysotile in
sample 8 is similar in composition and appearance to that found as a
trace phase in the dust samples.

SEM Figure 1 is a scanning electron microscope (SEM) image of a
representative portion of sample 22, collected from an area near the
World Trade Center. SEM Figure 2 is an image of a representative portion of
sample 3 collected near Battery Park. Both images show abundant glass
fibers along with particulate debris. SEM Figure 3 is an image of a bundle
of chrysotile asbestos from sample 8. The chrysotile was found as a
trace constituent in two of the samples analyzed so far by SEM/EDS and
in several others analyzed by XRD. SEM Figure 4 is a SEM image of gypsum
and/or anhydrite particles consistent with dry wall material.
Representative analyses of some of these phases are given in Table
SEM-1. Images from other samples can be seen in the Integration of Results section.

Sample 36 was recovered from an indoor location near the Trade Center
complex and had not been affected by rain as were the outdoor samples.
This sample was thoroughly analyzed for metal-rich particles to better
understand the types of materials from which metals might be leached
(see leachate studies section). Images
of several metal-rich particles can bee seen by clicking
here.

SEM Figure 1. Secondary electron SEM image of a representative portion of
sample 22. The image shows abundant glass fibers (red arrows) and other
materials as described above. The round object at the right side of the
image is a glass sphere with a composition similar to the fibers (see
Table SEM-1). Spheres like these are typical components of slag wool.

SEM Figure 2. Secondary electron SEM image of a representative portion of
sample 3 collected from Battery Park. The image shows abundant glass
fibers and other materials as described above.

SEM Figure 3. SEM image of a chrysotile bundle (center) and glass fibers
from sample 8. Similar bundles have been identified at trace levels in
several of the dust samples.